Generate a callable for autoloading package with target-dir

Proposal for composer/composer#925 : generates a callable that
can be passed to AnnotationRegistry to autoload annotation classes.

As @schmittjoh pointed out, a good solution to the issue would
be to have something to pass to the AnnotationRegistry. I've added
the generation of a simple file that return a callable for that.

Not sure about the file naming but this seems a pretty decent and simple solution to me